The present invention relates to a high-density ferrite member and a ferrite beads composition therefor as well as to a method for the preparation of the ferrite beads composition. More particularly, the invention relates to a high-density ferrite member having usefulness as a constituent of electric and electronic devices and a ferrite beads composition used for the preparation of the ferrite member by compression molding as well as a method for the preparation of such a ferrite beads composition.
As is well known, various types of ferrite materials in general are widely employed as a material for the preparation of ferrite members having usefulness as a structural or functional part of a great variety of electronic devices such as electric insulation members and others by utilizing their excellent properties in respect of high heat resistance, chemical stability, mechanical strength, electric insulation, magnetic properties and others.
Such a ferrite member is prepared in most cases by the dry-process compression molding of a ferrite powder-based composition comprising, for example, fine particles of the ferrite material and a binder compound to give a compression-molded body which is then subjected to a sintering heat treatment. Namely, ferrite particles are blended with a binder compound and water to give an aqueous slurry which is spray-dried in a spray drier to give a ferrite beads composition suitable for dry-process compression molding. Alternatively, such a ferrite beads composition is prepared by repeating drying and oscillating extrusion molding for granulation of a mixture of the ferrite particles and an aqueous solution of the binder compound.
In order to be used satisfactorily in the compression molding, the ferrite beads composition is required to have good flowability to ensure rapid and complete filling of a metal mold cavity for compression molding and good collapsibility of the beads under compression from the standpoint of the quality of the ferrite members as well as the productivity of the compression molding process. Proposals are made in Japanese Patent Kokai 5-159918 and Japanese Patent Publication 7-17460 on a method for the preparation of a ceramic beads composition having good flowability and exhibiting good collapsibility of the beads in which the aqueous slurry for the preparation of ceramic beads is admixed with a dispersing agent. Further, a proposal is made in Japanese Patent Publication 3-31660 and Japanese patent Kokai 10-59776 in which a ceramic beads composition is prepared by undertaking a measure to decrease segregation of the binder compound. Japanese Patent Kokai 10-29871 discloses a ceramic beads composition having good moldability in compression molding for the manufacture of sintered ceramic bodies having excellent mechanical properties, in which ceramic particles are blended in an aqueous slurry with a combination of three kinds of binder compounds including a polyvinyl alcohol, a polyethyleneglycol and a higher fatty acid ester of a polyhydric alcohol such as sorbitan stearate and the like.
Though effective to some extent when the ceramic beads composition is prepared by the method of spray-drying granulation of the aqueous slurry, these prior art methods are not very satisfactory when the oscillating extrusion granulation method is applied to the preparation of the ceramic beads composition from an aqueous slurry of the ceramic particles. In addition, these prior art methods have several problems in respect of irreproducibility of the density of the ceramic member prepared from the ceramic beads composition, releasability of the compression-molded body from the metal mold, crack formation in the ceramic member due to the phenomenon of so-called springback of the molded body and durability of the metal mold although certain improvements can be accomplished thereby in the flowability of the ceramic beads composition and collapsibility of the ceramic beads under a relatively low compressive force.